1. Field of the Invention
The present invention relates to a method for driving a planar light source device, a method for driving a color liquid crystal display (LCD) device assembly, a method for driving a light emitting diode (LED), and a pulse-width modulating method.
2. Description of the Related Art
In a color LCD device, liquid crystal itself emits no light. Thus, a planar light source device (backlight) is placed on a rear surface of the color LCD device so as to directly light the color LCD device. In the color LCD device, each pixel includes three sub-pixels: a red light emitting sub-pixel; a green light emitting sub-pixel; and a blue light emitting sub-pixel. By operating a liquid crystal cell constituting each sub-pixel as a kind of light shutter (light valve), that is, by controlling light transmittance of each sub-pixel, light transmittance of illuminating light (e.g., white light) emitted from the planar light source device is controlled, whereby an image is displayed.
Conventionally, a planar light source device in a color LCD display device assembly evenly lights an entire display area at constant luminance. Another planar light source device having a configuration different from that of the above-described planar light source device is known, as disclosed in Japanese Unexamined Patent Application Publication No. 2005-17324. This planar light source device includes a plurality of planar light source units, and distribution of illuminance varies in a plurality of display area units constituting the color LCD device. The planar light source device including a plurality of planar light source units may be called a “split-driven planar light source device” for convenience. Each of the planar light source units constituting the split-driven planar light source device disclosed in Japanese Unexamined Patent Application Publication No. 2005-17324 includes a red LED (light emitting diode), a green LED, and a blue LED. By mixing red light emitted from the red LED, green light emitted from the green LED, and blue light emitted from the blue LED, white light having a high chromatic purity can be obtained, and the white light is used as illuminating light.
The LED emits heat while being driven. Even under the same condition, variation occurs in a Vf characteristic as a result of heat emission, so that light output from the LED reduces. A reduction rate is different in the respective red, green, and blue LEDs. Particularly, light output from the red LED reduces significantly. This causes variation in a so-called white balance (color temperature) of white light, which is obtained as illuminating light by mixing light emitted from the red, green, and blue LEDs.
In the technique disclosed in Japanese Unexamined Patent Application Publication No. 2005-17324, the amount of driving current detected by a driving current detecting unit is fed back to a drive control unit. The amount of the fed back driving current is compared with a predetermined amount of current. On the basis of the comparison result, a drive control signal is changed so as to control the amount of light emitted from each of light emitting devices corresponding to three colors, whereby the white balance of a displayed image is controlled.
On the other hand, in the split-driven planar light source device, each of the planar light source units is controlled on the basis of the following method. This is disclosed in Japanese Unexamined Patent Application Publication No. 11-109317, for example. That is, maximum luminance in the planar light source unit is represented by Ymax, and a maximum value (specifically, 100%) of light transmittance (aperture ratio) of a liquid crystal cell constituting sub-pixels in a display area is represented by Ltmax. When the planar light source unit has the maximum luminance Ymax, light transmittance (aperture ratio) of a liquid crystal cell constituting a pixel to obtain luminance y0 in a display area unit is represented by Lt0. In this case, luminance of the planar light source unit (light source unit luminance Y0) is controlled so that y0·Lt0=Y0·Ltmax is satisfied. Thus, in a case where LEDS constituting the planar light source unit are driven in pulse-width modulation (PWM), pulse-width modulation control to obtain the light source unit luminance Y0 may be performed. That is, in the pulse-width modulation, a pulse-width modulation unit clock is represented by CLunit, an ON time is represented by tON, an OFF time is represented by tOFF, and a value of a pulse-width modulation output signal is represented by S. In this case, when the light source unit luminance Y0 is to be obtained, values S of three types of pulse-width modulation output signals for ON/OFF control of the red, green, and blue LEDs constituting the planar light source unit (a value SR of a pulse-width modulation output signal for ON/OFF control of the red LED, a value SG of a pulse-width modulation output signal for ON/OFF control of the green LED, and a value SB of a pulse-width modulation output signal for ON/OFF control of the blue LED) may be determined so that expressions “tON+tOFF=constant value tConst” and “tON=CLunit×S” are satisfied. The value of the pulse-width modulation unit clock CLunit is invariable.